Fine projection structure and fabricating method thereof

ABSTRACT

Dispose a fine metal particle on a semiconductor substrate. By heat-treating this in a vacuum, a constituent element of the semiconductor substrate is dissolved into the fine metal particle to form a solid solution, resulting in further formation of a homogeneous liquid phase (liquid droplet) composed of semiconductor-metal. By annealing this, the constituent element of the semiconductor substrate is precipitated from the semiconductor-metal liquid droplet. Thus, a fine projection composite structure comprising a semiconductor substrate, a semiconductor fine projection epitaxially grown selectively at an arbitrary position on the semiconductor substrate, and a metal layer disposed selectively on the semiconductor fine projection, can be obtained. The metal layer can be removed as demands arise. Such a fine projection composite structure possesses applicability in, for instance, an ultra-high integration semiconductor device or a quantum size device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fine projection composite structurein which a fine projection such as a semiconductor dot or asemiconductor/metal composite dot both of nanometer scale is formed on asemiconductor substrate, and a fabricating method thereof.

2. Description of the Related Art

An integration rate of a semiconductor device represented by a DRAM isincreasing year by year. For instance, the integration rate of a DRAMhas been heightened from 16 Mbit to 64 Mbit or 256 Mbit, and furtherdevelopment of a semiconductor device of the integration rate of morethan Gbit is under way. Such a high integration of a semiconductordevice has been achieved through reduction of a unit element size downto sub-micron order. For miniaturization of the unit size, developmentin lithography technology has largely contributed. In addition toimprovement of lithography technology, improvement of an elementstructure is also in progress.

Concerning the lithography technology, due to improvement of i-lineexposure technique and positive photo-resist, 0.5 μm rule correspondingto 16 Mbit-DRAM is being put into practical use. Further, due todevelopment of the exposure technique using KrF excimer laser which cancorrespond to 0.25 μm rule, 64 Mbit-DRAM is being mass produced and thepractical use of 256 Mbit-DRAM is under way. Further, correspondence to0.18 μm rule due to improvement of the exposure technique employing KrFexcimer laser or development of the exposure technique employing the SORlight is under way. However, the limit of present lithography technologyis considered to be about 0.1 μm rule. Therefore, in order to achievefurther higher integration, it is desired to be realized in the future aunit element size of nanometer scale.

Further, a quantum-size device is attracting attention as a candidate offuture LSI technology. Realization of a new device utilizing aquantum-size effect or a tunnel effect, for example, such as a quantumwire device or a quantum dot device which makes use of a wire or a dotstructure of which sectional dimension is at the same degree with aquantum mechanical wavelength of an electron, a resonant tunnellingeffect device or a resonant tunnelling element utilizing a quantum welland so on, are being tried.

In order to develop a new device which positively utilizes a quantumeffect, a characteristic dimension of an element should not remain in aphase wavelength (0.1 to 1 μm) order, namely in a mesoscopic region, butshould be brought into an electron wavelength order (10 to 100 nm),namely in a microscopic region. Further, in order to utilize moreeffectively a quantum effect device, ultra-miniaturization of the unitelement size itself of, for instance, 10 to 100 nm, more preferably lessthan 10 nm, is required. However, it is far beyond the presentlithography technology level.

As described above, research and development of the quantum size deviceand the like which are expected as candidates of a ultra-highintegration semiconductor device or future LSI technology are under way.To materialize such a fine device, a unit element size of nanometerscale is required to be achieved. Therefore, a technology enabling toobtain with reproducibility a semiconductor dot or a semiconductor/metalcomposite dot, which is necessary for a ultra-high integrationsemiconductor device or a quantum-size device, is desired to bedeveloped.

SUMMARY OF THE INVENTION

Therefore, the objective of the present invention is to provide a fineprojection structure of nanometer scale, which enables to realize a highintegration semiconductor device or a quantum-size device, andfabricating method thereof.

A first fine projection composite structure of the present inventioncomprises a semiconductor substrate, a fine projection consisting of asemiconductor grown selectively on the semiconductor substrate with anepitaxial relation to the semiconductor substrate, and a metal layerdisposed selectively on the fine projection.

A second fine projection structure of the present invention comprises asemiconductor substrate and a fine projection consisting of asemiconductor grown selectively on the semiconductor substrate with anepitaxial relation to the semiconductor substrate.

A producing method of a fine projection composite structure of thepresent invention is comprising of a process of disposing a fine metalparticle on a semiconductor substrate, a process of heat-treating thesemiconductor substrate on which the fine metal particle is disposed ata temperature higher than the temperature where a constituent element ofthe semiconductor substrate is incorporated into the fine metal particleto form a solid solution in vacuum atmosphere, and a process ofannealing for growing the constituent element of the semiconductorsubstrate epitaxially with respect to the semiconductor substrate from asolid solution phase in which the constituent element of thesemiconductor substrate is dissolved into the fine metal particle toform a solid solution.

In the present invention, a heat treatment is given to a semiconductorsubstrate being disposed with fine metal particles, at first, at atemperature where a constituent element of the semiconductor substrateis incorporated into the fine metal particles to form a solid solutionin a vacuum atmosphere, further, at a temperature equal or more thanthat where the constituent element of the semiconductor substrate andthe fine metal particles dissolve to form a homogeneous liquid solidsolution phase. By annealing from this liquid solid solution state, theconstituent element of the semiconductor substrate can be precipitatedfrom the liquid solid solution phase.

Through precipitation from such a liquid phase, the constituent elementof the semiconductor substrate grows epitaxially with respect to thesemiconductor substrate. Further, the constituent element of thesemiconductor substrate and the fine metal particle separate. Therefore,according to the size and the position of the initial fine particles,semiconductor fine projections can be formed epitaxially with respect tothe semiconductor substrate. In addition, even after the constituentelement of the semiconductor substrate is precipitated, the fine metalparticles remain on the semiconductor fine projections as metal layers.Therefore, a fine composite structure having a hetero-junction interfacebetween a semiconductor layer having a projection shape (semiconductorfine projection) and a metal layer can be obtained.

The shape of a semiconductor fine projection can be, for example, atrapezoid-like shape or variation thereof. The fine projection of thetrapezoid-like shape or modification thereof can be reduced to 1 μm orless in diameter at the maximum portion thereof, more preferably to 100nm or less. The fine projection of the trapezoid-like shape ormodification thereof can be made to 50 nm or less in diameter at theminimum portion thereof, more preferably to 10 nm or less. The size ofthe metal layer existing at the upper portion of semiconductor fineprojection can be made nearly identical. Further, through elimination ofthe metal layer at the upper portion, only a semiconductor fineprojection can be obtained. According to such a fine projection, a unitelement size of nanometer scale, which is required in, for example, aultra-high integration semiconductor device or a quantum-size device,can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D are diagrams schematically showingone embodiment of fabricating processes of a fine projection compositestructure of the present invention and a fine projection compositestructure obtained thereby.

FIG. 2 is a diagram schematically showing a structure of another fineprojection composite structure of the present invention.

FIG. 3 is a diagram schematically showing TEM observation results of afine projection composite structure fabricated in embodiment 1 of thepresent invention.

FIG. 4 is a diagram showing a result of an EDX analysis of A portion ofthe fine projection composite structure shown in FIG. 3.

FIG. 5 is a diagram showing a result of an EDX analysis of B portion ofthe fine projection composite structure shown in FIG. 3.

FIG. 6 is a diagram showing a result of an EDX analysis of B portion ofthe fine projection composite structure shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments carrying out the present invention will bedescribed with reference to the drawings.

FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D are diagrams schematically showingone embodiment of producing processes of a fine projection compositestructure of the present invention. At first, as shown in FIG. 1A, afine metal particle 2 is disposed on a semiconductor substrate 1.

As a semiconductor substrate 1 and a fine metal particle, a combinationof a semiconductor and a metal, which, from a thermodynamic point ofview, forms a solid solution at an elevated temperature and separates atlow temperature, can be employed. When a Si substrate is employed as thesemiconductor substrate 1, for instance, as a constituent member of thefine metal particle 2, Au, Ag, Al and the like can be employed. Inaddition, when a Ge substrate is employed as a semiconductor substrate1, as a constituent member of the fine metal particle 2, Zn, Cd, Au, Ag,Al can be employed. Like this, various combinations of semiconductorsand metals, which form solid solutions at an elevated temperature andseparate at a low temperature, can be used.

When forming a fine metal particle 2, the surface of a semiconductorsubstrate 1 should be made sufficiently clean. On the surface of such asemiconductor substrate 1, the fine metal particle 2 is formed underdecompression or vacuum state. The formation method of the fine metalparticle 2 is not restricted to the particular one but a method capableof forming the fine metal particles 2 on the semiconductor substrate 1held at room temperature can be employed. If a fine metal particle 2 isformed on a heated semiconductor substrate 1, it is likely that areaction layer or the like is formed at the interface between thesemiconductor substrate 1 and the fine metal particle 2 and is likely toadversely affect on the later processes. As a method forming fine metalparticles 2, a gas phase condensation method of metal can be cited.

The particle size of the fine metal particle 2, as will be describedlater, can be a size with which the fine metal particle can incorporatea semiconductor element (Si atoms, for instance) diffusing on thesemiconductor substrate 1 during heat-treatment, resulting in formationof a semiconductor-metal solid solution. To be specific, the diameter ofthe fine metal particle 2 is preferred to be 1 μm or less, morepreferable to be 100 nm or less. When an initial particle size of thefine metal particle 2 is too large, during formation process or heattreatment process thereof, it is likely that a reaction layer or thelike is formed at the interface between the semiconductor substrate 1and the fine metal particle 2. The interface reaction layer tends toadversely affect on the later processes and to disturb the precipitationof the semiconductor atoms from the semiconductor-metal solid solutionphase.

Next, to the semiconductor substrate 1 disposed with the fine metalparticles 2, heat treatment is carried out at a temperature higher thanthat where a constituent element of the semiconductor substrate 1 candissolve into the fine metal particle 2 to form a solid solution in avacuum. As illustrated in FIG. 1B, when the semiconductor substratedisposed with the fine metal particle 2 is heat treated in a vacuum,during the temperature elevating process, a rapid diffusion of itsconstituent atoms 1a occurs on the surface of the semiconductorsubstrate 1. The diffusing atoms (semiconductor atoms) 1a areincorporated into the fine metal particle 2. Thus, a semiconductor-metalsolid solution phase is formed.

In particular, by carrying out the heat treatment at a temperaturehigher than that where a liquid phase, in which the fine metal particle2 and the semiconductor atoms 1a are homogeneously dissolved, can beformed, a homogeneous semiconductor-metal solid solution liquid phase,that is, a semiconductor-metal liquid droplet 3 can be obtained. Theheat-treatment temperature, when the semiconductor substrate 1 and thefine metal particle 2 form an eutectic, is at least its eutectictemperature or more. Anyway, it can be a temperature higher than orequal to that where a semiconductor-metal liquid droplet 3 can beformed. Here, a semiconductor-metal solid solution phase is preferred tobe a liquid phase but can be a pseudo-liquid phase where semiconductoratoms and metal atoms are rapidly diffusing.

After formation of a liquid droplet 3 consisting of asemiconductor-metal solid solution liquid phase, this is annealed. Asillustrated in FIG. 1C, during annealing, constituent atoms of thesemiconductor substrate 1 precipitate from within a semiconductor-metaldroplet 3 to a solid-liquid interface between the liquid droplet 3 andthe semiconductor substrate 1, and precipitate with an epitaxialrelation with respect to the semiconductor substrate 1. Precipitation ofthe constituent atoms of the semiconductor substrate 1 are considered tobase on the size effect of the semiconductor-metal liquid droplet 3, theheat treatment temperature, the cooling speed and so on. Though thecooling rate is different depending on constituent material, about 2K/min or less is preferable.

As described above, the constituent atoms of the semiconductor substrate1 gradually precipitate from the semiconductor-metal liquid droplet 3,and precipitate maintaining an epitaxial relation with respect to thesemiconductor substrate 1, thus, a semiconductor layer 4' growsprojectingly between the semiconductor-metal liquid droplet 3 and thesemiconductor substrate 1. That is, the constituent atoms of thesemiconductor substrate 1 grow due to liquid phase epitaxy from thesemiconductor-metal liquid droplet 3. On the semiconductor substrate 1,a projection-like semiconductor layer 4' is gradually formed.

And, by completing the precipitation of the semiconductor atoms from thesemiconductor-metal liquid droplet 3 within the cooling process, asillustrated in FIG. 1D, a semiconductor fine projection 4 can be formedalmost completely separated from metal matrix (metal fine particle 2).Besides, the fine metal particle 2 remains as a metal layer 5 at theupper portion of the semiconductor fine projection 4. Thus, on thesurface of the semiconductor substrate 1, a fine composite projection 5of 2 layered structure, which is composed of the semiconductor fineprojection 4 possessing a selective and epitaxial relation, and a metallayer 2' disposed selectively on the semiconductor fine projection 4,can be formed.

The semiconductor fine projection 4 and the metal layer 2' in the finecomposite projection 5 can be completely separated. The shape of thesemiconductor fine projection 4, since the semiconductor atoms growthrough gradual precipitation from the semiconductor-metal liquiddroplet 3, its cross-section becomes a projection like having a nearlytrapezoid shape. The semiconductor fine projection 4 becomes a truncatedcone shape, for instance. The semiconductor fine projection 4, dependingon the initial size of the fine metal particle 2, can be made 1 μm orless in diameter at its maximum portion and about 50 nm or more indiameter at its minimum portion. The size of the semiconductor fineprojection 4 can be further reduced to 100 nm or less in diameter at itsmaximum portion and 10 nm in diameter at its minimum portion. The metallayer 2' can be made to have, for instance, the maximum diameter nearlyequal to the diameter at the minimum portion of the semiconductor fineprojection 4 and to have a cross-section of a nearly triangle shapepossessing such the minimum diameter as 1 nm or less.

Thus, by applying a fabricating method of the present invention, on anarbitrary position of a semiconductor substrate 1, a fine compositeprojection 5 of a 2 layered structure, which has a semiconductor fineprojection 4 which is about 100 nm or less (more preferably 50 nm orless) in diameter at its maximum portion and is epitaxially grownselectively with respect to the semiconductor substrate 1, and a metallayer 2' which is disposed selectively on the semiconductor fineprojection 4 and is nearly completely separated from the semiconductorfine projection 4, can be formed. In other words, a nanometer scalecomposite dot 5 having a hetero-junction interface composed of almostcompletely separated semiconductor/metal can be obtained. When the sizeof the initial fine metal particle 2 is small such as 10 nm or less, themetal layer 2' also grows epitaxially with respect to the semiconductorfine projection 4.

The size of the semiconductor fine projection 4 can be controlledthrough the size of the initial fine metal particle 2 or the heattreatment temperature. Further, through control of the disposingposition of the fine metal particle 2, the formation position of thesemiconductor fine projection 4 can be controlled on the surface of thesemiconductor substrate 1. Through the use of such a fine compositeprojection 5 of the 2 layered structure, a unit element size of, forexample, nanometer scale can be made to be realized. It is remarkablyeffective for realizing an ultra-high integration semiconductor deviceor a quantum size device. Other than these, various kinds of finedevices can be materialized.

When only a semiconductor fine projection 4 is necessary, a metal layer2' can be removed. By undergoing such a process, as illustrated in FIG.2, on an arbitrary position of the surface of the semiconductorsubstrate 1, a semiconductor fine projection 4, the diameter of which atthe maximum portion is 100 nm or less (more preferably 50 nm or less)and which is epitaxially grown selectively with respect to thesemiconductor substrate 1, can be obtained. A fine projection consistingonly of such a semiconductor fine projection 4 is also effective whenrealizing an ultra-high integration semiconductor or a quantum sizedevice.

In the following, concrete embodiments of the present invention will bedescribed.

Embodiment 1

At first, a Si(111) single-crystal substrate (non-doped, a₀ =0.5431 nm)is prepared. After the Si(111) single-crystal substrate is treated bychemical cleaning, to remove a native oxide layer and to obtain ahydrogen-terminated Si surface, the Si substrate was dipped in a dilutedHF solution (2 weight %) for 30 seconds. Following, pre-treated Si(111)single-crystal substrate was mounted in a vacuum chamber with abackground pressure of less than 1×10⁻⁶ Torr.

Then, on the surface of the above described Si(111) single-crystalsubstrate, Au fine particles were deposited. The Au fine particles weregenerated with a gas phase condensation method. That is, by evaporatingAu of 99.99% purity in an Ar gas atmosphere, the Au fine particles weredeposited on the Si(111) single-crystal substrate. Ar gas pressure wasset at 6 Torr to obtain Au fine particles of about 10 nm in diameter.These Au fine particles were deposited on the Si single-crystalsubstrate at room temperature. Diameter of the obtained Au fineparticles was observed with a TEM and was found to be about 10 to 15 nm.

Next, the Si single-crystal substrate deposited with the Au fineparticles was heat-treated in a high vacuum chamber of less than 1×10⁻⁸Torr. The temperature of the substrate is elevated at a rate of 15 K/minup to 1073K and maintained at this temperature for 30 min. Then, thesubstrate was gradually cooled at a rate of -2 K/min down to roomtemperature.

The structure and the composition of the above described heat treatedspecimen were evaluated with a high resolution transmission electronmicroscope (HRTEM, JEOL-2010) and an energy dispersive X-rayspectrometer (EDX, Oxford-Link ISIS). These results are shown in FIG. 3,FIG. 4, FIG. 5 and FIG. 6. FIG. 3 is a diagram schematicallyillustrating a cross-sectional TEM image of the specimen afterheat-treatment. TEM image was taken from <110> direction of the Sisingle-crystal substrate. FIG. 4, FIG. 5 and FIG. 6 show EDX spectra ofeach portions of the specimen after heat-treatment.

From a schematized diagram of a cross-sectional TEM image illustrated inFIG. 3, it is obvious that, on the surface of the Si single-crystalsubstrate 11, a fine dot of a 2 layered structure is formed. In the finedot 12 of the 2 layered structure, the lower layer portion 13 possessesa truncated cone-like shape, the diameter of its minimum portion wasabout 10 nm, and that of the maximum portion was about 30 nm. The upperportion 14 possesses the maximum diameter of about 15 nm and aprojection like shape of which the minimum diameter at the tip portionis 1 nm or less. Here, in order to evaluate the compositions of eachlayers 11, 13, 14, EDX spectra were obtained. The EDX analysis wascarried out with an electron beam of a diameter converged to less than 5nm. The evaluated positions are shown in FIG. 3 as point A, point B andpoint C.

FIG. 4 is an EDX spectrum taken at the point A of FIG. 3, FIG. 5 is anEDX spectrum taken at the point B of FIG. 3, FIG. 6 is an EDX spectrumtaken at the point C of FIG. 3. From the results obtained at the pointA, as illustrated in the spectrum of FIG. 4, peaks of only Si areapparent. Thus, it is obvious that, even after the heat-treatment, theSi(111) single-crystal substrate 11 maintains its state.

From the results obtained at the point B, as shown in the spectrum ofFIG. 5, it is obvious that the lower portion (neck portion) 13 of a finedot 12 of a 2 layered structure is formed of only Si. In addition, sincethe peak of Au in the spectrum of FIG. 5 is in the range of error, thislayer can be regarded as a Si single crystal layer.

Actually, in the TEM image schematically illustrated in FIG. 3, thelattice image of Si is clearly obtained at the lower portion 13 of thefine dot 12 of the 2 layered structure. In addition, this TEM imageclearly shows that the lower portion (Si layer) 13 of the fine dot 12maintains an identical crystal direction with respect to the Si(111)single-crystal substrate 11. Thus, the truncated cone-like shaped Silayer (Si dot) 13 of about 10 nm in the minimum diameter has epitaxiallygrown with respect to the Si(111) single-crystal substrate 11 on thesurface of the Si(111) single-crystal substrate 11.

Further, from the results obtained at the point C, as shown in FIG. 6,in the upper portion 14 of the fine dot 12 of the 2 layered structure,it is found that Au is a dominant component, though trivial Si beingfound.

Thus, the fine dot 12 is confirmed to be constituted of a Si dot 13grown epitaxially with respect to the Si(111) single-crystal substrate11, and a Au layer 14 which is disposed on the Si dot 13 and isconstituted of almost Au alone separated from the Si dot 13. That is, aSi--Au nanometer scale composite dot possessing an almost completelyseparated Si/Au interface was obtained.

The above described Si--Au nanometer scale composite structure isconsidered to be formed through the following liquid phase epitaxy. Atfirst, by carrying out the heat treatment in a high vacuum with respectto the Si substrate deposited with Au fine particles, Si atoms diffusingon the surface of the Si substrate are incorporated into the Auparticles to form Si--Au liquid droplets. This is evident from that theheat treatment temperature is sufficiently higher than the eutectictemperature of Si--Au of 643K.

By annealing after formation of the liquid droplets obtained by mixingSi and Au in liquid phase, Si atoms precipitate at the liquid-solidinterface between Si--Au liquid droplet and Si, and, during coolingprocess, grow epitaxially on the Si substrate. By completing suchprecipitation of Si in the course of the cooling process, Si dots areformed almost completely separated from Au fine particles, and Au layersremain on the Si dots.

Further, when the Au layer 14 of the upper portion in the abovedescribed Si--Au nanometer scale composite dot 12 is removed, only Sidot 13 which is epitaxially grown with respect to the Si(111)single-crystal substrate 11 was obtained.

As evident from the above described embodiments, according to thepresent invention, an epitaxial semiconductor dot or asemiconductor-metal composite dot of nanometer scale can be obtained.These remarkably contribute in realizing an ultra-high integrationsemiconductor device or a quantum-size device.

What is claimed is:
 1. A fine projection composite structurecomprising:a semiconductor substrate; a fine projection consisting of asemiconductor which is grown selectively on the surface of thesemiconductor substrate with an epitaxial relation with respect to thesemiconductor substrate, the fine projection having a trapezoid-likeshape in its cross section; and a metal layer disposed selectively onthe fine projection.
 2. The fine projection composite structure as setforth in claim 1:wherein, the fine projection of the trapezoid-likeshape is 1 μm or less in diameter at its maximum portion, and is 50 nmor less in diameter at its minimum portion.
 3. The fine projectioncomposite structure as set forth in claim 1:wherein, the fine projectionof the trapezoid-like shape is 100 nm or less in diameter at its maximumportion, and is 10 nm or less in diameter at its minimum portion.
 4. Thefine projection composite structure as set forth in claim 1:wherein, themetal layer is a triangle-like shape in its cross section.
 5. The fineprojection composite structure as set forth in claim 1:wherein, themetal layer comprises a metal material which forms a solid solution witha constituent element of the semiconductor substrate at an elevatedtemperature, and separates at a low temperature.
 6. The fine projectioncomposite structure as set forth in claim 1:wherein, the semiconductorsubstrate and the fine projection contain Si, and the metal layercontains at least one kind selected from Au, Ag, and Al.
 7. The fineprojection composite structure as set forth in claim 1:wherein, thesemiconductor substrate and the fine projection contain Ge, and themetal layer contains at least one kind selected from Zn, Cd, Au, Ag, andAl.
 8. A fine projection structure comprising:a semiconductor substrate;and a fine projection consisting of a semiconductor which is grownselectively on the surface of the semiconductor substrate with anepitaxial relation with respect to the semiconductor substrate, the fineprojection having a trapezoid-like shape in its cross section.
 9. Thefine projection structure as set forth in claim 8:wherein, the fineprojection of the trapezoid-like shape is 1 μm or less in diameter atits maximum portion, and 50 nm or less in diameter at its minimumportion.
 10. The fine projection structure as set forth in claim8:wherein, the fine projection of the trapezoid-like shape is 100 nm orless in diameter at its maximum portion, and is 10 nm or less indiameter at its minimum portion.
 11. The fine projection structure asset forth in claim 8:wherein, the fine projection of the trapezoid-likeshape is 50 nm or less in diameter at its maximum portion, and is 10 nmor less in diameter at its minimum portion.